Carboxylic acids such as acetic acid, acrylic acid, and methacrylic acid are important intermediates/feedstocks for the chemical industry. For example, acrylic acid and its derivatives are perhaps the most versatile monomers for providing performance characteristics to thousands of polymer formulations, such as adhesives, adsorbents, paints, polishes, protective coatings, and sealants, to name a few.
The production of unsaturated carboxylic acids by oxidation of an olefin is well known in the art. Acrylic acid, for instance, may be commercially manufactured by the gas phase oxidation of propylene. It is also known that unsaturated carboxylic acids may also be prepared by oxidation of alkanes. For instance, acrylic acid may be prepared by the oxidation of propane.
Acrylic acid is currently produced from propylene by the gas-phase heterogeneous oxidation of propylene. The process contains two stages; the first stage requires the oxidation of propylene to acrolein using mixed metal oxides, such as Mo—Bi—Fe—W—Co—Si—K—On. The yield from this step is generally greater than 96%. The second stage requires the oxidation of acrolein to acrylic acid; a step that proceeds at a much lower temperature than the first stage. The catalyst employed in the second stage is Mo—V—W—Cu—Sb—On. The yield from this step is generally 99%. While clearly an efficient process, with rising crude oil prices, the cost of propylene has reached record levels, severely impacting the cost of this gas-phase oxidation process. As may be appreciated, a process for the production of acrylic acid by the oxidation of propane can potentially yield significant savings in manufacturing costs.
Acrylic acid is also one of the fastest growing commodity chemicals, with projected annual growth rate of about 4%; thereby requiring, on average, a new world-scale plant every year to keep up with demand. With current worldwide demand at about 3.4 million tons a year, the cost savings alone by using less expensive feed propane has the potential to revolutionize the industry. However, a suitable process for the oxidation of alkanes to unsaturated carboxylic acids that is commercially viable has yet to be achieved.
From both an economic and technological point-of-view, the cost of manufacturing acrylic acid is difficult to reduce. Moreover, the high growth rate in polypropylene usage has caused concern around the world that a propylene shortage may occur in the future, further driving up the cost of propylene. As such, significant research efforts have been aimed at developing new technologies that employ propane.
One impediment to the production of a commercially viable process for the catalytic oxidation of an alkane to an unsaturated acid is the identification of a catalyst having adequate conversion and suitable selectivity, thereby providing sufficient yield of the unsaturated acid end-product.
Candidate catalyst systems for propane oxidation to acrylic acid have included vanadium pyrophosphate (VPO) type catalysts, which have been used successfully in the industrial process for n-butane oxidation to maleic anhydride. Another is the class of heteropoly acids and their salts. The third is the multi-component mixed metal oxides, which, as indicated above, have been utilized in propylene oxidation to acrylic acid. Mixed metal oxides containing MoVTeNb have been proposed in various patents for propane oxidation. One such catalyst for use in the oxidation of propane to acrylic acid is said to be a mixed metal oxide catalysts of Mo—V—Te—Nb—O.
Nitriles, such as acrylonitrile and methacrylonitrile, have been industrially produced as intermediates for the preparation of fibers, synthetic resins, synthetic rubbers, and the like. The most popular method for producing such nitrites is to subject an alkene such as propene or isobutene to a gas phase catalytic reaction with ammonia and oxygen in the presence of a catalyst at a high temperature. Catalysts proposed for conducting this reaction include Mo—Bi—P—O catalysts, V—Sb—O catalysts, Sb—U—V—Ni—O catalysts, Sb—Sn—O catalysts, V—Sb—W—P—O catalysts and catalysts obtained by mechanically mixing a V—Sb—W—O oxide and a Bi—Ce—Mo—W—O oxide. In view of prevailing prices, attention has been directed to the development of a method for producing acrylonitrile or methacrylonitrile by an ammoxidation reaction, wherein a lower alkane, such as propane or isobutane, is used as a starting material and catalytically reacted with ammonia and oxygen in a gaseous phase in the presence of a catalyst.
U.S. Pat. No. 5,380,933 proposes a method for preparing a catalyst said to be useful in the gas phase oxidation of an alkane to an unsaturated carboxylic acid. A catalyst is proposed which is said to prepared by combining ammonium metavanadate, telluric acid and ammonium paramolybdate to obtain a uniform aqueous solution. Ammonium niobium oxalate is added and the water removed to obtain a solid catalyst precursor. It is suggested that the solid catalyst precursor can be molded into a tablet, sieved to a desired particle size and then calcined at 600° C. under a nitrogen stream to obtain a catalyst.
U.S. Pat. Nos. 6,642,174 and 6,914,150 each propose a catalyst that includes a mixed metal oxide said to be prepared by a sol-gel technique. The catalyst is said to be useful for the conversion of an alkane, or a mixture of an alkane and an alkene, to an unsaturated carboxylic acid by vapor phase oxidation, or to an unsaturated nitrile by vapor phase oxidation in the presence of ammonia.
U.S. Pat. No. 7,009,075 proposes a process for the selective conversion of alkanes to unsaturated carboxylic acids in a one-step process with a mixed metal oxide catalyst composition. The mixed metal oxide catalyst is said to have the general formula: MoVaNbbTecSbdMeOx, wherein Me is said to be optional and may be one or more selected from silver, silicon, sulfur, zirconium, titanium, aluminum, copper, lithium, sodium, potassium, rubidium, cesium, gallium, phosphorus, iron, rhenium, cobalt, chromium, manganese, arsenic, indium, thallium, bismuth, germanium, tin, cerium or lanthanum. It is said that the catalyst may be prepared by the co-precipitation of metal compounds, which are calcined to form a mixed metal oxide catalyst.
U.S. Pat. No. 7,019,169 proposes a process for preparing (meth)acrylic acid by conducting a saturated hydrocarbon precursor compound through a catalyst bed whose catalysts are said to have, as the active composition, a multimetal oxide, which has a specific X-ray diffractogram and contains the elements Mo and V, at least one of the elements Te and Sb, and also at least one of the elements from the group consisting of Nb, Ta, W, Ce and Ti, wherein the catalyst bed is interrupted by at least one catalyst bed whose catalysts are said to have, as the active composition, a multimetal oxide which contains the elements Mo, Bi and Fe.
EP 0,962,253 proposes a process for preparing a multi-metal oxide catalyst. The catalyst is said to be useful for the gas phase oxidation of alkanes to unsaturated aldehydes or carboxylic acids.
EP 1,090,684 proposes a catalyst said to be useful for oxidation reactions. The catalyst is said to be useful for the gas phase oxidation of alkanes, propylene, acrolein, or isopropanol to unsaturated aldehydes or carboxylic acids.
Despite these advances in the art, there is a continuing need for new catalysts and improved processes for the production of a carboxylic acid.